1. Field of the Invention
The present invention relates to a secondary battery. More particularly, the present invention relates to a cap assembly that improves a safety device for preventing explosion of a secondary battery as a result of an increase in an internal pressure of the same, and to a secondary battery utilizing the same.
2. Description of the Related Art
A secondary battery allows recharging and may have a small size with a large recharging capacity. The main types of secondary batteries include a nickel hydrogen (Ni—H) battery, a lithium (Li) battery, and a lithium-ion (Li-ion) battery. The lithium secondary battery may be made into various shapes, with the most common shapes being cylindrical and quadrilateral, typically used for lithium-ion batteries.
In the case of the lithium-ion battery, if overcharging occurs or if there is a short between a cathode plate and an anode plate of an electrode assembly, decomposition of electrolyte occurs in which lithium salt and an organic solvent are mixed in the area of the anode plate, and leakage of lithium metal in the area of the cathode plate occurs. As a result, battery characteristics may deteriorate and an internal short may occur. There is a particular concern that gas generated during overcharging may increase an internal pressure such that the battery explodes and/or catches fire.
In order to solve the above overcharging and shorting problems of the secondary battery, a safety device is provided in the battery of a conventional can-type lithium-ion battery. The safety device uses a shut-down separator, a PTC element for performing cutoff of current when temperature increases, or a safety vent for allowing the exhaust of pressure when the pressure in the battery increases.
Examples of such a safety valve are disclosed in U.S. Pat. No. 5,985,478. and Japanese Patent No. 11-329407. The disclosed structures are such that passage apertures formed in a cathode plate undergo riveting using rivets connected to a tab in a state where an insulation member and a gasket are positioned at upper and lower areas of the cathode plate. There are limits with the use of this structure with respect to the degree to which the passage apertures are sealed. For example, in the case of large capacity secondary batteries, since a large amount of gas is generated within the battery, the degree to which the passage apertures are sealed is significantly reduced.
Another example of a safety device is disclosed in Japanese Patent No. 11-219692. The disclosed safety device disclosed uses a thin metal plate to seal holes of a cap plate. However, when the safety device receives an external physical shock, a crack generates in a safety vent. Therefore, reliability is an issue with this structure.
Japanese Patent No. JP 11-025935. discloses a safety device produced in an upper-directional prominence and depression configuration. A dual-layered metal thin plate is used in the structure, making the manufacturing process complicated and expensive.
The above mentioned safety devices do not have a mechanism to absorb pressure when internal pressure is increased by gas generated in the battery during charging and discharging. This inability to increase safety by the lack of a structure to absorb pressure is of particular concern for large capacity/high output secondary batteries, such as those used in HEVs (Hybrid Electric Vehicles).